Perhaps the most publicly visible products from the Intergovernmental Panel on Climate Change (IPCC) reports are the projections of future climate change. Aside from the evidence that humans are responsible for recent global warming, the projections are what make the nightly news. Maybe that’s why those projections are also the target of so much argument and, all too often, misrepresentation. When it comes to climate change, the globe isn’t the only thing that gets heated.

With a final draft of the physical science portion of the next IPCC report released a couple weeks ago, it’s worth taking a little time to examine what, exactly, the new projections entail.

Projections 101

What the heck is a projection anyway? A psychic might claim to predict the future, your local TV meteorologist forecasts the weekend weather. But climate model projections are different things entirely. First, since future climate change depends on many types of human-caused emissions and landscape changes, several projections are generated based on a range of potential changes. The relevance of a projection depends on how closely the emissions scenario it relies on matches what actually takes place.

Second, the climate modelers recognize that there are no crystal balls that would allow them to foresee things like major volcanic eruptions (which cause short-term cooling), changes in the behavior of the Sun, or the status of El Niño in 2028. Global average surface temperature varies a bit from year to year because of interactions within the climate system. The IPCC report states, “For projections, no attempt is made to predict the evolution of the internal variability. Instead, the statistics of this variability are included as a component of the uncertainty associated with a projection.”

That is, the projections represent the longer-term trends that can be obscured by “noise.” This isn’t because climate models somehow omit natural variability. The IPCC projections are created by averaging simulations from many different climate models. Each simulation exhibits year-to-year wiggles due to variability, so no two simulations will be identical. But when many simulations are averaged together, those ups and downs of the variability cancel out.

The purpose of these projections is to illustrate the impact of our actions. They show what the climate will look like if no action is taken to curb greenhouse gas emissions, or if we instead pursue aggressive reductions. These different potential futures are termed scenarios.

The scenarios used in the last IPCC report have been replaced by a simpler set of four, which are known as “Representative Concentration Pathways” (RCPs). They’re named based on how much more energy is trapped by greenhouse gases in 2100—2.6 Watts per square meter of the Earth’s surface, or 4.5, 6.0, or 8.5. While RCP8.5 is basically a “business-as-usual” scenario where emissions continue to grow at the present rate, the other scenarios include varying degrees of emissions reductions. In the two middle scenarios, greenhouse gas concentrations are stabilized—before the end of the century in one case, after in the other.

In the lowest scenario (RCP2.6), very aggressive changes result in concentrations not only stabilizing very soon but decreasing afterward. This assumes that we develop and deploy technology to actually remove CO2 from the atmosphere—an option that is not currently feasible. Previous IPCC reports did not include a scenario where that takes place.

Let’s talk numbers

It takes a while before the differences between these scenarios become significant. There is natural variability as well. As a result, the projections of average global surface temperature don’t diverge until around 2050. So to understand how things are likely to progress for the next few decades, we don’t need to choose a scenario.

The report says that global temperature averaged over the time period from 2016 to 2035 is likely to be 0.3-0.7°C (that’s about 0.5-1.0 °F) warmer than the average from 1986 to 2005. Comparing 20-year averages may be an unwieldy way to think about it, but it’s important when you're dealing with a combination of long-term trends and short-term variability. (An equivalent comparison for the past several decades yields about 0.47°C warming.) The projection equates to 0.12-0.42°C of warming per decade through 2035.

The chart below, taken from the report, shows the climate model simulations underlying this determination. The spaghetti noodle mess gives you a sense of the range of variability exhibited by the models. By 2050, the highest and lowest emissions scenarios (red and blue lines) begin to separate from the jumble, but there is still some overlap.

In the latter part of the 21st century, the effect of the differences between emissions scenarios becomes clear. Looking again at 20-year averages, we can compare the last two decades of the century with the 1986 to 2005 baseline. The low-end scenario, where CO2is removed from the atmosphere, results in a world that is 0.3–1.7°C warmer. The two middle scenarios produce 1.1–3.1°C of warming. The business-as-usual scenario, however, warms the planet by 2.6–4.8°C (about 4.7–8.6°F).

Enlarge/ Projections for two emissions scenarios are shown in this graph, with the shaded areas above and below each line indicating 90 percent of the variation between individual simulations.

IPCC AR5 Summary for Policymakers Figure 7

While we tend to focus on atmospheric temperatures, around 90 percent of the heat being trapped by increasing greenhouse gases goes into the oceans. Because of the size of the oceans and the fact that water requires lots of energy to change temperature, ocean temperature changes are smaller than those in the atmosphere. That change is greatest at the water surface, and shrinks the deeper you go.

Depending on which of the four scenarios you look at, the report projects about 1–3°C of ocean surface warming by 2100. At a depth of one kilometer (0.6 of a mile), the warming ends up being 0.5–1.5°C.

Warning: uneven road ahead

It’s important to remember that climate scientists don’t expect the world to steadily warm a little each year without any deviation from the trend. This is at least somewhat apparent from the spaghetti noodle chart showing model simulations. But just how variable is global temperature in those simulations?

In a commentary on surface atmosphere temperatures in the last decade, Indian Institute of Science climate scientist Govindswamy Bala provides a clear visualization for an answer. For the chart below, he calculated moving 10-year averages (shifting one year at a time) for simulations that used one of the middle emissions scenarios.

Unsurprisingly, some 10-year periods have trends higher or lower than the average (shown in kelvins per decade, which is equivalent to °C per decade). If you look carefully, you’ll notice that periods where the trend is near zero (or even cooling) aren’t uncommon. That’s the natural variability the models produce for a scenario that results in about 2°C warming by the end of the century.

While the scenarios have changed from the last IPCC report, the projected warming is basically the same. This is what our knowledge of Earth’s climate tells us will happen because of human activity. That’s the direction the ship is headed. Though it’s slow to turn, that ship does have a rudder.